Producing bleached sulphite pulp with high strength properties from hardwoods



United States Patent O V PRODUCING BLEACHED SULPHITE PULP WITH HIGH STRENGTH PROPERTIES FROM HARD- WOODS Erik K. M. Hiigglund, Stockholm,

Mo Och Domsjii Aktiebolag, company of Sweden Sweden, assignor to Omskoldsvik, Sweden, :1

No Drawing. Application November 6, 1950, Serial No. 194,395

The present invention relates to a process for producing from hardwood bleached sulphite pulp in a high yield and with high strength properties.

It is known to digest hardwood with practically neutral sodium sulphite (NazSOa) by cooking at a temperature of l70180 C. In this process a high yield is obtained of a pulp which is bleachable and has good strength properties. process is very expensive owing to the employment of sodium sulphite which cannot be economically recovered.

The present invention is based on the surprising observation that it is possible to produce from hardwood notwithstanding its short fibre length-a bleached sulphite pulp with exceedingly high strength properties, high brightness and exceedingly high purity in a very high yield, by combination of a special pulping process for digestion in one stage at low temperature and a special bleaching process.

In accordance therewith the present invention relates to a process for producing from hardwood a bleached sulphite pulp in a high yield, for instance about 50-60%, calculated on the weight of the wood used as absolutely dry, and a correspondingly high content of hemicellulose amounting to at least 30% of that contained in the original wood, and with high strength properties, for instance corresponding to a tensile strength of the order of magnitude of l0,000-11,000 m., and a burst factor of about 70 in case of a beating degree of about 45 SR, and a brightness of at least 80% G. E. 1

The process according to the invention is mainly characterized in that the hardwood is digested with a calcium bisulphite cooking liquor at a temperature of about 110 C. for such a time that a pulp is obtained which is disintegrated and subjected to a bleaching in two or more stages while using chlorine dioxide or chlorite as bleaching agent for the final bleaching for high brightness.

When carrying out the process according to the invention a cooking liquor with a base content of 12-20% by weight of CaO and a total S02 content of 6.5-8% by weight is advantageously used. The digested and disintegrated pulp is suitably not allowed to dry before the bleaching. As indicated above, the bleaching is carried out in at least two stages, in many cases preferably in three or four stages, for example in the first stage with chlorine, in the next with a mixture of chlorine and bypochlorite and in one or two final stages with chlorine dioxide or chlorite with intermediate washings with alkali solution and final washing with water.

As an example of the manner of carrying out the bleaching may be mentioned that the pre-bleaching in the first stage is suitably carried out with elementary chlorine, for example in a quantity of l.3 the Roe-number. Hereafter follows a washing with alkali, for example with an aqueous solution of sodium hydroxide, preferably containing 2% NaOH, calculated on the pulp, suitably at a temperature of about 4060 C. Hereupon may follow a final bleaching with chl'orite or chlorine dioxide in a second bleaching stage. However, if

However, there is the drawback that the water.

the bleaching is to be performed in three or four stages, the second bleaching stage maybea weaker chlorination carried outwith elementary chlorine+calcium or sodium hypochlorite in which the quantity of active chlorine is preferably 1.5'2.5%, calculated on the weight of the pulp. Hereafter follows a. further washing with alkali, for example with a solution of sodium hydroxide, preferably containing 0.5-1.5 NaOH, calculated onthe Weight of the pulp, suitably at a temperature of 40-60" C., whereupon the pulp is washed free fromalkali with water, for example at a temperature of 50-60 C. After these two preliminary bleaching stages a final bleaching is carried out with an acidified chlorite solution, for example of a pH of 4-5, preferably ata temperature of 6065 C., a pulp concentration of about 5% and a quantity of chlorite corresponding to about 1.5%, calculated onthe pulp as active chlorine, whereupon suitably a washing is effected. Thi's bleaching stage may advantageously be followed by an additional bleaching stage, also in this case with an acidified chlon'te solution, for example at the same pH, temperature and pulp concentration, with a quantity of chlorite suitably corresponding to about l.01.2%, calculated on the pulp as active chlorine, whereupon the pulp is washed with water. I

For carrying out the final bleaching, instead of' a chlorite, also chlorine dioxide prepared in advance may be used, either in aqueous solution or in'gas form, in which latter'c ase the chlorine dioxide is diluted with an inactive gas. The bleaching is carried outin acid milieu, for example ata pH inth'epulp suspension of 4 to 5, the'quantity cf chlorine dioxid'e suitably'corresponding to 1.5 calculated on thernass as active chlorine. After this treatment with chlorine dioxide the pulp is suitably washed with The interruption of the digestion at an early stage in connection with the above mentioned bleaching process results in the production of a bleached pulp possessing exceedingly high strength properties,-as indicated above, said strength properties surprisingly in certain respects being even higher than those of the unbleached pulp. The yield 'of this high quality bleached. pulp is, as mentioned above, 50 to 60%, calculated on the dry weight of the wood used.

Theprocess according to the invention is further illustrated below in five non-limitative examples.

Example I 110 C. whereupon the digestion was carried out at this temperature for 12 hours.

Hereby was obtained a pulp with a Roe-number of 7.2, a tensile strength of 10,430 m. and a, burst factor. of 61, in a yield of 58.8%, calculated'bn the-weight ofthe wood used.

This pulp was subjected to a bleaching in two stages, in the first stage with chlorine and inlthe second stage with chlorine dioxide, with an intermediate Washing with NaOH solution and a final washing with water.

The bleached pulp thus obtained had a tensile strength of 11,260 m.', a burst factor of 73-, afolding number of 7,800 and a brightness of 85.4% G. E. The yield of bleached pulp was 87.6% ofthe unbleached pulp, e. g. 51.5%, calculated on the weight of the wood originally used.

Example 2 Chips. of aspen (Popu'lus tremuloides) were subjected to adigesti'on as described in Example 1 with the difference' that the'peri'od of digestion at C. was only 5.5 hours.

2,719,788 Patented Oct. 4, 1955 pubescen-s) were subjected to The pulp obtained had a Roe-number of 10.9, and the yield was 66.4% of the wood used. In case of a beating degree of 45 SR the tensile strength of the pulp was 9,400 m.

The pulp was bleached in two stages as in Example 1 and had then in bleached condition a tensile strength of 9,100 m., a folding number of 3000 and a brightness of 87.7% G. E. The yield of bleached pulp was 57.2%, calculated on the weight of the dry wood.

Example 3 Chips of birch (Betula verrucosa) were digested with a cooking acid containing 6.6% S02 and 1.0% G210, a ratio acidtwood being 3.7. The cooking temperature was raised during 4 hours to 120 C. whereupon the cooking'was carricdout at this temperature for 4 hours. The yield of pulp hereby obtained was 55.6%.

This pulp was subjected to a bleaching in two stages, as indicated in Example 1, the active chlorine consumption being totally 10.3%. Hereby was obtained a bleached pulp in a yield of 50.0%, calculated on the wood used, said mass having a brightness of 85.5% G. E. In case of a beating degree of 40 SR the pulp had a tensile strength of 10,900 m., a burst factor of 71, a folding number of 6000 and a tear factor (Elmendorf) of 66.

Example 4 Chips of beech (Fagus silvatica) were digested with an acid of the same composition as in Example 3. The digestion was carried out by cooking at a temperature of 110 C. for 5 hours. The yield of pulp thus obtained was 70.0%.

Said pulp was then bleached in the same manner as in Example 1, the total consumption of chlorine being 20%. The yield of bleached pulp thus obtained was 50.5%, said pulp having a brightness of 88.0% G. E. The strength properties of said pulp were as follows: Tensile strength 10,100 m., first factor 63, folding number 2900, tear factor (Elmendorf) 81.

Example 5 Chips of oak (Quercus) were digested by cooking with a cooking liquor of the same composition as in Example 3 at a temperature of 110 C. for 4 hours. Hereby was obtained a pulp in a yield of 71.5%.

This pulp was subjected to a three stage bleaching, with chlorine in the two first stages and addition of totally 2.5%, counted on the pulp weight, of HCl in the chlorination stages. In the third stage the bleaching was carried out with chlorite (chlorine dioxide). The total amount of chlorine consumed in the bleaching was In this manner a bleached pulp was obtained in a yield of 51.8%. Said pulp had a brightness of 89.0% G. E., a tensile strength of 8800 m., a burst factor of 53, a folding number of 2000 and a tear factor (Elmendorf) of 70, in case of a beating degree of55 SR.

As equivalent to a cooking liquor with calcium as base it is also possible, according to the invention, to use a cooking liquor containing ammonium as base.

I claim:

1. A process for producing bleached hardwood sulphite pulp in a high yield and with high strength properties, which consists substantially in cooking hardwood in a suitable state of subdivision with calcium bisulphite cooking liquor at a temperature of approximately 110 C. to 120 C. for from about 4 to 12 hours suflicient to yield a digested and disintegrated pulp in an amount of 50 to 60%, based on the weight of the wood used as absolutely dry, and having a corresponding high hemi cellulose content amounting to at least 30% of that contained in the original wood, dispersing the disintegrated pulp in water, subjecting the pulp in a first bleaching stage to a chlorination with elemental chlorine in a quantity of about 1.3 times the Roe-number of the pulp, washing the pulp with an alkali solution at elevated temperature, subjecting in a second bleaching stage the chlorinated pulp in a pulp concentration of about 5%, in an acid medium, to a final bleaching with a bleaching agent of the group consisting of a chlorite and chlorine dioxide in a quantity corresponding to about 1.5%, based on the pulp, of active chlorine, and washing the bleached pulp to yield a bleached pulp having a tensile strength of approximately 10,000 to 11,000 m., a burst factor of 60 to 70 corresponding to a beating degree of 40 to SR and a brightness of at least 80% G. E.

2. A process for producing bleached hardwood sulphite pulp in a high yield and with high strength properties, which consists substantially in cooking hardwood in a suitable state of subdivision with calcium bisulphite cooking liquor, containing from about 1.2 to 2.0% byweight of CaO and a total S02 content of from about 6.5 to 8% by weight, at a temperature of approximately 110 C. to 120 C. for from about 4 to 12 hours sulficient to yield a digested and disintegrated pulp inan amount of to based on the weight of the wood used as absolutely dry, and having a corresponding high hemicellulose content amounting to at least 30% of that contained in the original wood, dispersing the disintegrated pulp in water, subjecting the pulp in a first bleaching stage to a chlorination with elemental chlorine in a quantity of about 1.3 times the Roe-number of the pulp, washing the pulp with an alkali solution at elevated temperature, subjecting the so-treated pulp to a second weaker chlorination with elemental chlorine and a hypochlorite of the group consisting of calcium and sodium hypochlorite, in an amount corresponding to a quantity of active chlorine of about 1.5 to 2.5%, based on the weight of the pulp, washing the chlorinated pulp with an alkali solution at elevated temperature, Washing the pulp tree from alkali with water at elevated temperature, and subjecting the so-treated pulp in a pulp concentration of about 5%, in acid medium, to a final bleach ing in at least one stage with a bleaching agent of the group consisting of a chlorite and chlorine dioxide in a quantity corresponding to about 1.5%, based on the pulp, of active chlorine, and washing the bleached pulp to yield a bleached pulp having a tensile strength of approximately 10,000 to 11,000 m., a burst factor of 60 to corresponding to a beating degree of 40 to 45 SR and a brightness of at least G. E.

3. A process for producing bleached hardwood sulphite pulp in a high yield and with high strength properties, which consists substantially in cooking hardwood in a suitable state of subdivision with calcium bisulphite cooking liquor, containing from about 1.2 to 2.0% by Weight of CaO and a total S02 content of from about 6.5 to 8% by weight, at a temperature of approximately C. to C. for from about 4 to 12 hours sufficient to yield a digested and disintegrated pulp in an amount of 50 to 60%, based on the weight of the wood used as absolutely dry, and having a corresponding high hemicellulose content amounting to at least 30% of that contained in the original wood, dispersing the disintegrated pulp in water, subjecting the pulp in a first bleaching stage to a chlorination with elemental chlorine in a quantity of about l.3 the Roe-number of the pulp. washing the pulp with an alkali solution at elevated temperature, subjecting the so-treated pulp to a second weaker chlorination with elemental chlorine and a hypochlorite of the group consisting of calcium and sodium hypochlorite, in an amount corresponding to a quantity of active chlorine of about 1.5 to 2.5%, based on the weight of the pulp, washing the chlorinated pulp with a Weaker alkali solution at elevated temperature, washing the pulp free from alkali with water at elevated temperature, and subjecting the so-treated pulp in a pulp concentration of about 5% in acid medium to a final bleaching with a bleaching agent of the group consisting of a chlorite and chlorine dioxide, in a quantity corresponding to about 1.5%, based on the pulp of active chlorine,

washing the pulp so-treated, again bleaching by use of the same agent in a quantity corresponding to about 1.0 to 1.2%, based on the pulp, as active chlorine, and finally Washing the bleached pulp with water, to yield a bleached pulp having a tensile strength of approximately 10,000 to 11,000 m., a burst factor of 60 to 70 corresponding to a beating degree of to SR and a brightness of at least 80% G. E.

4. A process for producing bleached hardwood sulphite pulp in a high yield and with high strength properties, which consists substantially in cooking hardwood in a state of subdivision with calcium bisulphite cooking liquor having a base content of 1.2 to 2.0% by weight of CaO and a total S02 content of 6.5 to 8% by weight at a temperature of approximately 110 C. to 120 C. for from about 4 to 12 hours sufiicient to yield a digested and disintegrated pulp, dispersing the disintegrated pulp in water to obtain a fiber suspension, subjecting said fiber dispersion to a bleaching in at least one step with elemental chlorine in a quantity of about 1.3 Xthe Roenumber of the pulp and a final bleaching in at least one step with chlorine dioxide in a quantity corresponding to about 1.5%, based on the pulp, of active chlorine, and subjecting the bleached pulp to a final washing with water. to obtain a bleached pulp in a yield of to based on the weight of the wood used as absolutely dry, with a hemicellulose content amounting to at least 30% of that contained in the original wood and having a tensile strength of approximately 10,000 to 11,000 m., a burst factor of 60 to corresponding to a beating degree of 40 to 45 SR and a brightness of at least G. E.

References Cited in the file of this patent UNITED STATES PATENTS 1,599,489 Richter et al. Sept. 14, 1926 1,843,467 Traquair et al. Feb. 2 1932 2,019,598 Dreyfus Nov. 5, 1935 2,502,410 Cederquist Apr. 4, 1950 2,530,244 Jensen Nov. 14, 1950 OTHER REFERENCES Serial No. 216,133, Palazzo (A. P. C.), published May 4, 1943 (now abandoned).

Fries: Paper Trade Journal, May 23, 1946, pages 43-44; Tappi sec., pages 223-224.

Lockard: Chemical Abstracts vol. 41, 6401-g, 1947.

Russell: Paper Trade Journal, March 24, 1949, pages 23-26 (Ind. Dev. Sec., pages -108).

Dominion Forest Service Bull. 97, Canada, Dept. of Mines and Resources (1942), pages 29-36 and 64-69.

Jayme et al.: Chemical Abstracts, vol. 40, page 3897 (1946).

Richter: Chemical Abstracts, vol. 35, page 3439 (1941).

Manufacture of Pulp and Paper, 3d edition, volume III, sec. 4, page 95, published by McGraw-Hill, New York (1937).

Pulp and Paper Manufacture, volume 1, page 312, published by McGraW-Hill, New York (1950).

Feola et al.: Paper Trade Journal, September 26, 1946, page 67.

Moore et al.: Pulp and Paper Mag. of Canada, Conv. Issue, 1946, page 114.

Hagglund, Paper Industry, July 1931, page 512. 

1. A PROCESS FOR PROCUDING BLEACHED HARDWOOD SULPHITE PULP IN A HIGH YIELD AND WITH HIGH STRENGTH PROPERTIES, WHICH CONSISTS SUBSTANTIALLY IN COOKING HARDWOD IN A SUITABLE STATE OF SUBDIVISION WITH CALCIUM BISULPHITE COOKING LIQUOR AT A TEMPERATURE OF APPROXIMATELY 110* C. TO 120* C FROM ABOUT 4 TO 12 HOURS SUFFICIENT TO YIELD A DIGESTED AND DISINTEGRATED PULP IN AN AMOUNT OF 50 TO 60%, BASED ON THE WEIGHT OF THE WOOD USED AS ABSOLUTELY DRY, AND HAVING A CORRESPONDING HIGH HEMICELLULOSE CONTENT AMOUNTING TO AT LEAST 30% OF THAT CONTAINED IN THE ORIGINAL WOOD, DISPERSING THE DISINTEGRATED PULP IN WATER, SUBJECTING THE PULP IN A FIRST BLEACHING STAGE TO A CHLORINATION WITH ELEMENTAL CHLORINE IN A QUANTITY OF ABOUT 1.3 TIMES THE ROE-NUMBER OF THE PULP, WASHING THE PULP WITH ANALKALI SOLUTION AT ELEVATED TEMPERATURE, SUBJECTING IN A SECOND BLEACHING STAGE THE CHLORINATED PULP IN A PULP CONCENTRATION OF ABOUT 5%, IN AN ACID MEDIUM, TO A FINAL BLEACHING WITH A BLEACHING AGENT OF THE GROUP CONSISTING OF ACHLORITE AND CHLORINE DIOXIDE IN A QUANTITY CORRESPONDING TO ABOUT 1.5%, BASED ON THE PULP, OF ACTIVE CHLORINE, AND WASHING THE BLEACHED PULP TO YIELD A BLEACHED PULP HAVING A TENSILE STRENGTH OF APPROXIMATELY 10,000 TO 11,000 M., A BURST FACTOR OF 60 TO 70 CORRESPONDING TO A BEATING DEGREE FO 40 TO 45* SR AND A BRIGHTNESS OF AT LEAST 80% G.E. 